Ultrasonic apparatus calibration procedures fall into three categories, namely Transducer Zero Compensation, Material Velocity Calibration and Zero Offset Calibration. Presently, “Two Point Calibrations”, which is widely known in the art, utilizes the combination of “Zero Offset Calibration” and “Material Velocity Calibration.”
“Two Point Calibration” for thickness measurement instruments typically involves the process of adjusting an ultrasonic inspection device (UT device) so that it takes measurement on known-thickness test objects for a particular material, using a particular transducer at a particular temperature. In most cases, Material Velocity and Zero Offset Calibration may be combined using a thick and a thin calibration block of the same material, which is referred to as “Two Point Calibration”.
Material Velocity Calibration is typically performed using a thick test block of known thickness that is fabricated out of the same material to be measured, by measuring the time of flight of the ultrasonic signal that travels from the front surface to the back surface of the test material. This calibration needs to be completed for each batch of test objects.
Zero Offset Calibration is typically performed using a thin test block of known thickness made of the same material to be measured, by measuring the time of flight of the ultrasonic signal that travels from the front surface to the back surface of the test material. This calibration only needs to be performed once for each new transducer and material combination.
An existing conventional Two Point Calibration process for a given transducer typically involves the following steps:                1) Select a calibration block comprising a few sub-blocks with different but known thicknesses. Select two sub-blocks, with the thinner one called “thin block” and the thicker one called “thick block”. The thicknesses of both thin block and thick block are known. The pertinent parameters of the transducer selected to be calibrated are either recalled from the UT device's memory or provided by the operator.        2) Determine T1, the measured time of flight (TOF) of the thick block, by using the UT device and the selected transducer. T1 is the TOF measured for the ultrasonic signal to travel from the front surface to the back surface and back to the front surface of the thick block.        3) Provide H1, the known thickness of the thick block, to the UT device.        4) Determine T2, the measured time of flight (TOF) of the thin block, by using the UT device and the selected transducer. T2 is the TOF measured for the ultrasonic signal to travel from the front surface to the back surface and back to the front surface of the thin block;        5) Provide 112, the known thickness of the thin block, to the UT device.        6) Lastly, the UT device performs the Two Point Calibration calculations and stores the results.        
However, it is quite frequent that the UT device acquires an erroneous T2 for the thin block an account of factors such as incorrect gain or signal noise. The calibration would be therefore erroneous when an operator mistakenly accepts the erroneous T2. This has been problematic particularly for inexperienced operators who might unknowingly perform erroneous calibrations, which subsequently produce erroneous inspections. For experienced operators, erroneous readings slow down the calibration process, since the operator needs to stop and verify the calibration manually, which decreases productivity. Moreover, if there is not a waveform display on the UT device to view the signal during the calibration session, the operator has no means to determine if T2 is correct.
The accuracy of non-destructive testing (NDT) is well known to be critical for many industries.
Existing efforts are exemplified in U.S. Pat. No. 3,554,013 to Jerry Berg which deploys hardware circuitry to minimize the problems caused by erroneous calibration due to wrong signal detection. However, the hardware solution is comparatively not cost effective and adaptable and suffers from instability with thermal drift.
Thus, given the existing problems and tried efforts, there is a critical need to automatically remove erroneous calibration signals, especially for ‘thin block’ or “Zero Offset Calibration” to improve the inspection certainty, accuracy and to increase productivity.